(1) Field of the Invention
The present invention relates to an image forming apparatus for recording image information by means of laser beam irradiation on a film of the long roll type being transported by drive of a stepping motor.
(2) Description of Related Art
There is known an image forming apparatus of the above-noted type comprising a housing, a film delivery section for a film of the roll type placed inside the housing, a film accommodating section disposed in the housing for forming a film transport passage together with the film delivery section, a main drive unit disposed on the film transport passage in the housing and for driving the film at a fixed speed and an image information recording section constituted by a laser optical unit for applying a laser beam onto the film in a width direction of the film being driven and transported by the main drive unit. The main drive unit includes a stepping motor and a platen roller driven by the motor for transporting the film contacting an outer circumferential face of the roller.
In the image forming apparatus constructed as above, two-dimensional images are formed in series on the long-length film by a laser beam scanning operation in the film is width direction while the film being transported.
In order for the above apparatus to obtain an image of good quality, it is essential that the film transport operation be carried out at a constant and equal speed. According to one known arrangement attempting to meet this requirement, which is shown in FIG. 10, there is provided a microstep driver for sub-dividing one step unit of the stepping motor powered ON and OFF by a CPU into smaller (micro) step units, thereby minimizing such transport irregularities or errors.
The stepping motor employed in the above-described conventional image forming apparatus has an inherent disadvantage to be described next. That is, the stepping motor is in a magnetically balanced, i.e., stable state when one of its rotor pairs is opposed to a stator. Thus, if a predetermined amount of film transport is completed in the middle of one-step rotation of the stepping motor, there occurs a magnetic instability such that either pair of the rotors will stop only after automatically proceeding or receding in its rotation to become opposed to the stator in order to regain the stability. Therefore, if the stepping motor is re-energized for the next film transport, this film transport operation is inadvertently carried out from the above-noted stabilized position of the motor which has shifted from the previous position assumed by the motor before being re-energized. However, if an image is formed on a long-length film as used e.g. in a page printer, its frame pitch must be maintained exactly constant. In such case, the stepping motor is disadvantageous because of its above-noted instability which necessarily results in inconsistency in the frame pitch. This problem appears conspicuously in the case of a microfilm where an image is recorded in micron unit precision to be enlarged for use thereafter.
Next, the frame pitch will be defined. As shown in FIG. 9, images are formed in series in respective adjacent frames F on the film. The frame pitch P is defined here as a pitch between an adjacent pair of frames, i.e.,a distance: P from a leading edge of one frame to a leading edge of the adjacent frame next in order in the direction in which the film is transported.